This invention relates to gas-sensing semiconductor devices for detecting the presence and/or concentration of one or more gases.
It is known to fabricate a gas microsensor within a semiconductor device. In particular it is known to produce high-temperature metal oxide conductimetric sensors using alumina substrates and semi-manual production methods. Recently attempts have been made to manufacture silicon versions of such sensors employing a platinum heater integrated with a thin insulating membrane of oxide or nitride. Such sensors offer lower power consumption than conventional sensors when operating at 300° C. to 600° C. However the two part deposition of the membrane of such a sensor and the deposition of the metal heater layer sandwiched between the two membrane layers makes the fabrication process incompatible with integrated circuit technology. There has also been much interest in the development of MOSFET potentiometric sensors using catalytic gates, for example of palladium which run at temperatures of between 120° C. and 200° C. However such sensors will have limited application due to their inefficiency and relatively high cost.
WO98/32009 discloses a gas sensor comprising a semiconductor substrate, a thin insulating layer on one side of the substrate, and a thin semiconductor layer on top of the thin insulating layer. The sensor includes at least one sensing area in which the material of the substrate has been removed to leave a membrane formed by the thin insulating layer and the thin semiconductor layer, the or each sensing area being provided with a gas-sensitive layer and a heater for heating the gas-sensitive layer to promote gas reaction with the gas-sensitive layer. Furthermore the or each sensing area incorporates a MOSFET formed in the thin semiconductor layer and forming part of the heater and/or sensor, and the sensor provides an electrical output indicative of gas reaction with the gas-sensitive layer. However such a gas sensor can only operate at relatively low temperatures because the parasitic bipolar transistor inherent in any MOSFET structure can turn on at high temperatures. In addition, if Aluminum is employed as in the CMOS metal layers, there is a maximum temperature of 250° C. beyond which long term degradation by electromigration or stress can occur.
There is a need to make small, low-cost gas sensing devices that incorporate a micro-heater to elevate the temperature of a gas-sensitive layer (e.g. tin oxide) and have integrated electronic circuitry. Gardner J. W., Pike A., de Rooij N. F., Koudelka-Hep M., Clerc P. A., Hierlemann A. and Gopel W., 1995 Sensors and Actuators, B 26 135-139, “Integrated chemical sensor array for detecting organic solvents” have reported the use of platinum or doped polysilicon to form a resistive track. However, platinum is a material that is not compatible with CMOS technology, and polysilicon requires additional process steps and forms heaters that tend to lack long-term stability. As referred to above, WO98/32009 discloses the use of a MOSFET to form an active heater but, in silicon technology, the operating temperature of the device is limited to about 300 to 350° C. (see Udrea F., Gardner J. W., Setiadi D., Covington J. A., Dogaru T., Lu C-C. and Milne W. I., 2001 Sensors and Actuators, B 78 180-190, “Design and simulations of a new class of SOI CMOS micro hot-plate gas sensors”) and so is unsuited for, say, methane detection that requires an operating temperature of about 550° C.
The use of tungsten interconnects in high temperature CMOS processes is known. For example, in a paper by W. Yun, R. Howe, and P. Gray, “Surface micromachined, digitally force-balanced accelerometer with integrated CMOS detection circuitry,” Proc. of the IEEE Solid-State Sensor and Actuator Workshop '92, p. 126, 1992, there is a description of the use of tungsten as metallisation in place of aluminium, in order to allow further post-CMOS high temperature processing, which tungsten can withstand successfully.
Tungsten has also been used as high temperature metallisation in SOI CMOS processes (see, for example, “Tungsten metallisation for high-temperature SOI devices”, by J. Chen and J. P. Colinge, paper E-1.4, European Materials Research Society, Strasbourg, France, 1994, and Materials Science and Engineering).
However none of these references teaches the use of tungsten as a resistive heater in a CMOS compatible process.
It is an object of the invention to provide an improved high-temperature gas-sensing semiconductor device which can be produced at low cost using conventional bulk fabrication processes.